viktoryfeel

S4=b1×qn(n - степень) -1/q-1

q=b3/b2=1/4÷1/2=1/4×2/1=1/2

b1=b2/q=1/2÷1/2=1

S4=1×((1/2)⁴-1)÷1/2-1= -15/16÷(-1/2)=15/8